I guess we’re simply free to choose between serif and sans-serif as we please.
One of the things that makes this a difficult topic to analyze is familiarity: we generally are more comfortable reading the typefaces we are used to seeing.
So traditionally in print, serif fonts are used for body copy and sans serif for headlines. Studies from 30 years ago claimed serif was “easier” to read, but perhaps we were just used to it. Today on the Web, sans serifs are often used for body text and now many people think those are “easier” to read…
Personally I find line-length and leading (vertical space between lines) to be far more important for readability than the type of font.